The axons of newly born neurons extend long distances before reaching their targets in the vertebrate brain. Pioneering retinal ganglion cell (RGC) axons in Xenopus laevis make this 450 Mum journey unerringly through a variety of cellular terrains. How are these early axons guided? The present proposal specifically addresses this question by investigating the interactions of the tips of RGC axons with their microenvironment as they pioneer the pathway. The experiments propose three main lines of investigation: 1) Light and electron microscopy of dye-filled RGC axons will be used to characterize the morphology and the microenvironment of pioneering growth cones at different stages of their advancement along the optic pathway. 2)\ Intracellular injection of Lucifer Yellow into RGCs will reveal whether their growth comes become dye-coupled to other cells in the pathway. If such coupling does occur, antibodies that specifically block gap junctional conductance will be used to test whether intercellular communication is necessary for axonal navigation. 3) To find out when the embryonic nervous system first becomes permissive to axonal pathfinding, eye primordia will be transplanted to progressively younger embryos. The earliest stage that directed axonal growth is supported will be determined and it will be asked whether specific guidance structures, identified ultrastructurally or immunologically, are associated with the onset of these permissive conditions. These experiments will provide a better understanding of the factors that influence the behavior of growing axons in vivo and help to define the cellular basis of axonal navigation in the embryonic CNS.